The present invention relates to a process for producing briquetted and pressed (granular material and the use thereof for coloring building materials, such as concrete and asphalt, and organic media, such as paint systems, plastics and colored pastes.
The processing of pigment granules requires that the pigments be ground to primary particles in order to achieve the optimal color effect. The resulting powders formed create a large amount of dust and, owing to their finely-divided state, tend to adhere and stick in dosing plants. In the case of toxicologically harmful substances, during the processing precautions must therefore be taken to avoid danger to humans and the environment owing to the dust formed. But even in the case of safe, inert substances such as, for example, iron oxide pigments, avoidance of irritation due to dust is being increasingly demanded by the market.
The aim when handling pigments is accordingly avoidance of dust and improved dosing as a result of good flow properties, in order to achieve a color effect of even quality for use in building materials and organic media. This aim is more or less achieved by applying granulation processes to pigments. Here pelletizing and spray granulation are generally used. Compacting processes have hitherto been less suitable, owing to the limited dispersibility of the granular material obtained thereby.
In principle, in the case of pigments the market demands two diametrically opposing properties for the use of pigment granules: mechanical stability of the granular material and good dispersibility. The mechanical stability is responsible for good transport properties during transport between manufacturer and user as well as for good dosing and flow properties in the pigments in use. It is produced by high adhesive forces and is dependent, for example, on the quantity of binder or even on the compacting pressure during forming. On the other hand, the dispersibility is influenced by a thorough grinding prior to granulation (wet grinding and dry grinding), by the mechanical energy during the incorporation (shear forces) and by dispersing agents, which immediately lower the adhesive forces in the dry granular material during the incorporation into a medium. The use of larger quantities of dispersing agents in pigments is limited, however, owing to the cost ratio of additive to pigment. A high proportion of additive moreover causes a corresponding decrease in the coloring strength or in the scattering power. As the variations in coloring strength are generally less than.+-.5%, the use of additives is also restricted even if these are acting simultaneously as adhesion promoters and dispersing agents. Furthermore, the additives must not unfavorably alter the properties in use of end products such as building materials, plastics and paints: for example, the strength or the setting properties in concrete, the compressive strength or abrasion resistance in asphalt and the strength or the notch impact resistance in plastics and the elastic properties in elastomers (polymers).
Suitable production processes according to prior art for pigment granules are, for example, spray granulation (spray drying by disk or nozzle) and pelletizing (mixers, fluid-bed granulators, disks or drums).
The spray-drying granulation starts from pigment suspensions with the use of binders. Relevant processes are described in various protective rights; here water-soluble binders are used. Thus in DE-A 3 619 363, EP-A 0 268 645 and EP-A 0 365 046 the processes start from organic substances such as, for example, lignosulphonates, formaldehyde condensates, gluconic acids, sulphated polyglycol ethers, whereas in DE-A 3 918 694 and U.S. Pat. No. 5 215 583 the processes start from inorganic salts such as, for example, silicate and phosphate. A combination of spray granulation and pelletizing has also been described in EP-A 0 507 046. In DE-A 3 619 363 (column 3, lines 44-47) and EP-A 0 268 645 (column 7, lines 18, 19) the use of a compacting process is adopted. In this process a strong coherence of the particles is achieved by application of pressure, so that a good transportability but at the same time decreased properties of dispersibility are produced.
In EP-A 0 257 423 and DE-A 3 841 848, spray granulation using polyorganosiloxanes as hydrophobic, lipophilic additives is described. The spray-dryer mentioned generally leads to small particle sizes, that is, to a high proportion of fine material. This means that a significant proportion of the material is not obtained from the dryer as immediately usable granular material, but is first retained as fine material in the filter and has then to be returned to the process. The aftertreatment carried out to render the material hydrophobic, in the case of spray-dried products, results in granular material which flows very well but is exceptionally dusty.
EP-A 0 424 896 discloses the production of fine granular material low in dust, in a production operation in known intensive mixers. A low content of waxes in combination with emulsifier and wetting agents is used here by creating an aqueous dispersion. In the course of this water contents of from 20% up to over 50% are generally obtained. These granular materials must first of all be dried and separated from oversize and undersize material.
DE-A 31 32 303 describes inorganic pigment granules which are free-flowing and low in dust, which are mixed with binders rendered liquid by the action of heat and are granulated by a screening process with the use of a screening aid (pressure). About 10 to 20% of the throughput accumulates as fine material of&lt;0.1 mm.
EP-A 0 144 940 discloses pigment granules low in dust which, starting from filtration sludge containing about 50% water, through the addition thereto of 0.5 to 10% of surfactants as well as mineral oil or waxes liquefying at 50 to 200.degree. C., are mixed until lubrication point is reached. The procedure is carried out in intensive mixers, and if necessary the mixture is granulated and dried. Water is present in the end product in a quantity of 10 to 15%, which is disadvantageous for introduction into plastics.
Other processes are also limited in their application. Spray granulation, owing to the formation of droplets, requires the use of free-flowing and hence highly fluid suspensions. Consequently, for the drying process a greater quantity of water has to be evaporated than from highly pressed-out filtered pigment pastes during the frequently applicable fluid-bed drying. This leads to high energy costs. In the case of pigments previously produced by calcination, spray granulation involves an additional processing step with high energy costs. Moreover, in spray granulation a greater or lesser proportion of fine material accumulates, which has to be returned to the production unit.
Pelletizing, too, frequently exhibits disadvantages. Starting from pigment powder, it may be carried out in mixers under conditions of high turbulence, in the fluid-bed process or else by disk granulation and drum granulation. Common to all these processes is that the requirement for binder, in most case, water, is high, so that drying has to follow as an additional processing step. Here, too, granular materials of differing size are obtained, especially if insufficient binder is available for the quantity of powder or if the actual distribution is not optimal. Then a certain fraction may be too large for use as granular material, while on the other hand fractions which are excessively small and hence dust-forming are still present. A classifying of the granular material formed is therefore necessary, with oversize and undersize material being returned.
Disk granulation leads to granular materials having a wide particle size spectrum. Where this is undesirable because of the poor dispersibility of excessively large particles, the granulation process has to be monitored through intensive supervision by staff and the production of granular material has to be optimised by manual control of the quantity of grains. This is generally also followed by a classification and return of oversize and undersize material.
Extrusion processes from pastes lead to the formation of relatively solid granular materials during drying; owing to their size, these do not guarantee an optimal dispersibility.
DE-A 42 14 195 describes a process for coloring asphalt using inorganic pigment granules, with oils being used as binders. It is a simple granulation process.
In DE-A 4 336 613 and DE-A 4 336 612 inorganic pigment granules are produced from pigments by mixing with binders, compacting, rough grinding and granulating. The granular materials thus produced are not transportable satisfactorily by pneumatic means; during transportation a large quantity of dust is formed, which is undesirable.